A load dependent valve of this type is shown and described in Japanese Patent No. 59-26506 which is used for adjusting the applied brake force in accordance with the weight of the car which is dependent upon the number of passengers in the car with pneumatic suspension. The detailed description of the structure and operation will now be explained with reference to FIGS. 4 and 5 of the present application. As shown in FIG. 4, an inlet port 16 is connected to a suitable source of air pressure via conduit or line 16a. It will be seen that an outlet port 17 is connected to the brake cylinders via line 17a. It will be noted that a pair of inlet ports 18 and 19 are connected to a pair of separate air springs via lines 18a and 19a, respectively. The effective area of the control piston 4 is effectively twice as large as the effective area of the control piston 5. The two air spring pressures which are supplied to the air spring connection inlet ports 18 and 19 are averaged. A connecting rod 3 is attached to the pistons 4 and 5 and the upper end of the connecting rod 3 is arranged to contact the underside of a lower pivotal lever 7. The fixed end of the lower lever 7 is pivotally mounted on the main body of the load dependent valve so that it can rotate freely, and an adjustable biasing spring 10 exerts a force on the upper side of the other free end of the lever 7. The lower side of an exhaust valve rod 1 is cooperatively associated with a balance piston 2, and the lower end of the rod 1 contacts the upper side of an upper pivotal lever 6. As shown, the fixed end of the upper lever 6 is also pivotally mounted on the main body load dependent valve so that it can rotate freely, and an adjustable biasing spring 9 exerts a force on the underside of the other free end of the lever 6. The biasing spring 9 is adapted to maintain the brake pressure of the empty car when the air spring pressure PX becomes lower than the preset empty car pressure PA as noted in FIG. 5 when the air spring is damaged or for some other reason. This can be adjusted by an adjusting screw 11. The basing spring 10 is to allow the brake pressure to rise when the air spring pressure PX becomes higher than the pressure PN of the empty car. This can be adjusted by the second screw 12. A roller member 8 is sandwiched between the lower lever 6 and the upper lever 7, and it can be moved in a right or left direction from outside of the valve by any appropriate means. When roller 8 is moved to the left, the necessary force to move the air spring pressure upwardly is increased, and when the roller 8 is moved to the right, the force is decreased. In other words, the more the roller 8 moves to the left, the steeper the slope becomes between the empty car and the full car shown in FIG. 5.
When the air spring pressure PX becomes lower than the preset empty car pressure PN as a result of damage to the load dependent valve or for some other unknown reason, the second biasing spring 10 overcomes the air spring pressure acting on the control pistons 4 and 5 and the lower lever 7 and the connecting rod 3 to move downwardly, and the first biasing spring 9 acts on the exhaust valve rod 1 via the upper lever 6 completely independent of the air spring pressure so that the preset empty car brake pressure PC is obtained from the output 17.
When the air spring pressure PX is lower than the empty car pressure PN, the air spring pressure which acts on the control pistons 4 and 5 cannot push the second lever 7 upwardly against the force of the second spring 10. However, when the pressure becomes higher than the preset empty car pressure PN, the air spring pressure acting on the control pistons 4 and 5 overcomes the second biasing spring 10 and pushes the connecting rod 3 and the lower lever 7 upwardly. It also pushes the exhaust valve rod 1 via the roller 8 and the first lever 6 upwardly. When the exhaust valve rod rises, the air supply valve 13 also rises, so that the air supply hole 15 opens, and the compressed air is conveyed from the inlet port 16 which is connected to the air source to the outlet port 17 so that the output pressure increases. Then, the pressure acting on the top of the balance piston 2 also increases, and when it balances with the force acting from the lower direction on the exhaust valve rod 1, the air supply valve 13 and the exhaust valve rod 1 move down at the same time by the action of the return spring, and the air supply inlet 15 is closed.
When the air spring pressure PX decreases because of passengers getting off the train, the force acting on the exhaust valve rod 1 through the control pistons 4 and 5, the connecting rod 3, the lower lever 7, the roller 8, and the upper lever 6 also decreases. Thus, the exhaust valve rod 1 is pushed downwardly by the output pressure acting on the balance piston 2 so that the exhaust port 14 opens and the air in the outlet 17 is exhausted.
When the output pressure PY decreases, the pressure which acts on the balance piston 2 also decreases so that when it becomes slightly less than the force acting on the lower side the exhaust valve rod 1 is again pushed upwardly and contacts the air supply valve 13 wherein the exhaust outlet 14 is closed. Thus, as can be seen in FIG. 5, an output pressure PY which is proportional to the air spring pressure PX can be obtained, and the minimum brake force of the empty car can be maintained even if the air spring is defective and the air spring pressure decreases abnormally.
FIG. 5 is a graphic illustration of the characteristics of the output air pressure PY, the pressure at the outlet 17, in relation to the air spring pressure PX of the load dependent valve. As shown in FIG. 5. PN is the air spring pressure of the empty car, and the output air pressure PY is kept constant PC even if the air spring pressure PX becomes lower than this PN and the output air pressure rises with the specified slope adjusted by the roller 8 when it becomes larger than PN.
As described above, in the present load dependent valve, the slope can be changed, as shown by the phantom lines in FIG. 5, by varying the lever ratio which is dependent upon the position of the roller 8 so that the subject load dependent valve can correspond to the various condition of the car in which the weight of the car, air spring, and the required deceleration are different.
In the previous load dependent valve, the minimum output air pressure P and the empty car air spring pressure PN can be preset by adjusting the force of the first biasing spring 9 and the second biasing spring 10, and the characteristic of the output air pressure PY in relation to the air spring pressure PX can be changed by moving the position of the roller 8. However, the position of the levers and piston shifts during operation because of the slope of the first and the second levers 6 and 7 so that it is difficult to transmit the force accurately. Moreover, the contacting point of the roller 8 and the contacting lever portion are subject to wear, and therefore, the output air pressure is different from the initial output air pressure for a specified air spring pressure even if the roller is positioned at the same place after it has been in use for a long time. Accordingly, the sensitivity as well as the response capability becomes degraded.